Inboard mounted internal combustion engines used to drive small marine craft are almost invariably directly cooled by water. This is pumped from the water around the craft directly via a filter through the engine block and thence overboard. This may necessitate measures to prevent internal corrosion of the engine.
Land vehicles, apart from a few which are air cooled, employ indirect means whereby liquid coolant is circulated through the engine block from and back to a heat exchanger, that is a radiator, which is cooled by air directed through the exchanger.
The problem with amphibians is that the sole provision of direct water cooling is not feasible because the vehicle may be on land, and air intakes for indirect cooling radiators cannot always be located in convenient positions. Furthermore, the difficulty of mounting a radiator in a convenient position may result in precious space being taken up by air ducts or fans or both.
External water-water heat exchangers have been tried on amphibians, but have two notable drawbacks. One is that they are susceptible to damage, particularly from foreign objects; the other is that they inhibit marine performance and reduce hydrodynamic efficiency by breaking up the otherwise smooth contour of the underside of the hull.